Water flooding and surfactant flooding are processes well known in the art to recover the vast quantities of oil which remain in the formation after primary oil recovery operations. Designing new surfactants and surfactant systems of high oil recovery efficiency is a critical step toward advancing this technology.
To be effective in liberating oil from a petroleum reservoir, a surfactant must, in general, be able to reduce the interfacial tension between oil and aqueous reservoir fluid from around 30 dynes per centimeter to a few millidynes per centimeter or less. The mahogany sulfates referred to in this disclosure are able to accomplish this; however, their performance is limited to use in fresh or very slightly saline water. Since very few reservoirs normally contain fresh water as their aqueous phase, the use of mahogany sulfonates has been limited to experimental plots in which an attempt to replace the brine with fresh water was tried. The replacement process, with the accompanying need to dispose of the old brine, has proved ineffective. Also, the replacement of aqueous reservoir fluid with a fluid of less salinity has introduced deleterious changes to the reservoir, not all of which are completely understood.
Ordinary commercial non-ionic detergents are able to function in brine, but the minimum interfacial tension achieved by these is generally too high to be useful, being usually well over one dyne per centimeter.
The purpose of this invention has been to synthesize a surfactant which will give a low interfacial tension (5 millidynes per centimeter, or less) between reservoir oil and reservoir brine. In addition, it was intended that, by proper adjustment of surfactant composition, a specific member of a given category of surfactant could be made to suit the salinity of any reservoir.
Work on this surfactant has proceeded from certain definite theoretical premises:
(1) The surfactant will contain a hydrophobic part and a hydrophilic part. These will be opposite ends of a long molecule. The molecule will tend to orient at an interface with its hydrophobe in the oil and its hydrophilic portion in the aqueous phase. PA1 (2) The hydrophobic part should have, as nearly as possible, the same solvency as the reservoir oil. This means that the hydrophobe should have no tendency to clump together, rather, they should comingle freely with the oil molecules. An extremely unfavorable case would be that of a fluorocarbon hydrophobe. The fluorocarbon is not soluble in oil, and a surfactant made therefrom is ineffective. Even such differences as those between normal paraffin chains and "iso" (branched) chains prevent perfect solvency. The difference in solvency of normal and iso paraffin chains is quantified in "The Solubility of Nonelectrolytes", by Hildebrand and Scott, Third Edition, Dover Publications, Inc., New York. In the Appendix I "Selected Values of Solubility Parameters", the parameter .delta. for n-octane is 7.55, while the .delta. for 2,2,4 trimethyl pentane is 6.85. PA1 The hydrophilic part of the surfactant should be compatible with brine, even a hard brine, containing ions of calcium and magnesium. The poly(ethylene oxide) chain has this compatibility. The poly(ethylylene oxide) chain has the configuration: --CH.sub.2 --CH.sub.2 --O--CH.sub.2 --CH.sub.2 --O--CH.sub.2 --CH.sub.2 O--H. It can be produced with any degree of polymerization. The formula above contains three ethylene oxide units. The poly(ethylene oxide) chain owes its solubility in water to the fact that the oxygen atoms can hydrogen-bond to some of the water molecules. This bonding persists when the aqueous phase becomes a brine; however, two hindrances are brought into play. First, many water molecules are tied up in bonding to the ions of the salt (or salts). The water, in effect, becomes less concentrated, and the poly(ethylene oxide) chain must extend farther (be longer) in order to maintain the same bonding force it had in pure water. Secondly, many of the oxygen atoms of the chains are bonded with cations of the salt (or salts). Fortuitously, the latter effect is mitigated by the fact that the cations themselves are hydrated (bonded to water). Thus, the poly(ethylene oxide) chain retains a solvency which is compatible with brine. It is obvious, however, that, the more concentrated the brine, the longer must be the poly(ethylene oxide) chain used to generate a hydrophilic force which can compete with attractions at the hydrophobe, so as to hold the surfactant molecule at the interface. These conditions lead to the next statement. PA1 (4) The surfactant should have the correct hydrophile/lyophile balance (HLB). In other words, the hydrophilic portion should be large (long) enough to keep the hydrophobe from pulling the whole molecule into the oil. Likewise, the hydrophobe should be large enough to prevent the hydrophilic chain from pulling the molecule into the aqueous phase. The proper HLB leaves the molecule balanced at the interface. PA1 In connection with HLB, it should be noted that an increase in temperature, which causes a faster break-up of hydrogen bond, has a similar effect to that of an increased brine concentration in making the ethylene oxide chain less hydrophilic. Hence, at higher temperatures, a longer ethylene oxide chain must be used to generate the same hydrophilic force. PA1 (5) Since the surfactant molecule must stay in the interface in order to lower the interfacial tension it must be heavy enough that normal thermal perturbation do not displace it into one phase or the other. Short chain acids (caproic, capryllic, and capric, e.g.) are known to not even make useful soaps. From a typical soap-former, stearic acid (C.sub.18 H.sub.35 COOH) we get the C.sub.18 H.sub.37 -radical. However, this radical with suitable ethylene oxide addition to yield C.sub.18 H.sub.37 --[OCH.sub.2 CH.sub.2 ].sub.n OH does not have enough molecular weight to keep it in the interface so as to obtain millidyne/centimeter interfacial tensions. The octadecyl(C.sub.18 H.sub.37 -) radical has a molecular weight of 253, while the molecular weight considered effective in this application is in the range 300 to 500. PA1 (6) It can be noted that the first criteria for a waterflood surfactant are structural and thermodynamic in nature. From the method of use of such surfactants, there is also a requirement which is kinetic in nature; i.e., the surfactant must be able to move into and out of the surface in an unhindered manner. The surfactant, in use, is dissolved in the brine phase. As it contacts oil, fingering of oil occurs, along with emulsion and coacervate formation. These processes multiply greatly the interfacial surface and require that appropriate amounts of surfactant move into the surface. Whenever a massive bank of oil can be formed as it moves through the formation, interfacial surface must decrease, and surfactant must move out of the interfac. Alternatively, if produced at the well-head as an emulsion, it is desirable that the oil separate to the top in a resonable time, say 24 hours. Thus, the surfactant is not an emulsifier in the usual sense, producing, as it does, an unstable emulsion.
Research leading to the subject of this invention was directed toward the synthesis of a surfactant which would meet the six criteria heretofore outlined.
No particular attention was paid to meeting criterion (1), since the criterion mentioned there is a typical one for surfactants. In order to meet criterion (3), that of hydrophile solvency, it was necessary to go to the poly(ethylene oxide) chain, since it is the only feasible hydrophile group known which is compatible with strong brine. The use of the poly(ethylene oxide) chain makes it easy to meet criterion (4), since the chain can be synthesized to any needed number of units (n).
No means of achieving criterion (6) was known; its attainment was pure serendipity.
To attain the hydrophobe of perfect solvency, as per criterion (2), it was considered that there would be no better way to secure a hydrophobe compatible with an oil than to take the hydrophobe from the oil itself. At the same time, the molecular weight criterion (5) could be achieved through the utilization of a fraction of the oil of sufficiently high molecular weight.
In the actual synthesis of the detergent, there is a detail which has not been heretofore addressed, namely: the provision of a reactive group to the hydrophobe to which an ethylene oxide chain could be attached. In general, the reactive groups suited to this purpose are:
--OH--(either alcohol or phenol) PA0 .dbd.NH--secondary amine PA0 --NH.sub.2 --primary amine PA0 --CONH.sub.2 --amide PA0 --COOH--carboxyl PA0 --SH--thiol PA0 --COH--aldehyde glycerol or glyceride or saccharide
The common feature of these groups is that of a hydrogen atom which can be ionized very sparingly. Groups which ionize strongly, as the sulfate (--SO.sub.4 H) and sulfonate (--SO.sub.3 H) groups are not suitable.
An ideal source for hydrophobic material is found in the alkaline mahogany sulfonates, such as those produced by the Sonneborn division of Witco Chemical Company. These are produced as a by-product in the refining of white mineral oil. In the process, a higher-boiling fraction of a paraffin base crude is treated with fuming sulfuric acid or oleum. Portions of the crude, especially those molecules containing aromatic and heterocyclic rings are sulfonated and are dissolved in the acid phase. The acid phase, containing sulfonic acids, is separated from the reaction mixture and is washed with water. The water treatment separates the sulfonic acids into "green acids, which are soluble in the water and "mahogany acids" which are oil-soluble and remain in an oil layer. Neutralization of the oil layer gives the "mahogany sulfonates" of commerce.
The mahogany sulfonates of Witco are sold under the trade name "Petronate." Four of the most useful Petronates span an "equivalent weight" range of from 410 to 510. Considering that the --SO.sub.3 Na group has a molecular weight of 103, the hydrophobe molecular weight of the series runs from 307 to 407. For the first application of this invention, the medium weight Petronate HL (Equivalent Weight=440-470) was tried. Since a sulfonate cannot be treated with ethylene oxide, the first consideration concerned the feasibility of treating the Petronate with sodium hydroxide so as to produce a phenol (or possibly an alcohol). EQU RSO.sub.3 Na+NaOH.fwdarw.ROH+Na.sub.2 SO.sub.3
It was possible to do this at atmospheric pressure in the equipment available. The reaction was even carried to the melting point of sodium hydroxide (318.degree.) without loss of Petronate by boiling. This was an unexpected result which was of great convenience.
The second consideration involved the feasibility of reacting the phenol mixture with ethylene oxide without a difficult purification step to remove the by-product sodium sulfite. As attested by results presented here, it was possible to proceed without the purification step. (This was unexpected, but of great importance.)